The Prison Service is investigating after an undercover journalist got a job at the top-security jail housing Soham murders suspect Ian Huntley.
Ian Huntley is currently at HMP Woodhill
The News of the World has published photos of Mr Huntley in his cell, said to have been taken by reporter David McGee.
Mr McGee also claimed he had twice guarded Mr Huntley during his stint at Woodhill prison in Buckinghamshire, and had several conversations with him.
He claimed this was despite giving the authorities a false address, a bogus CV and false references - one from a convicted criminal.
A separate inquiry into the prison is already under way, after Huntley was taken ill after apparently taking a drugs overdose in a suicide attempt.
Mr McGee said he "simply called up" the prison and asked for a job.
"Within 13 weeks of starting training, and while I was still a rookie warder,
I was the sole guard minding Huntley," he wrote.
Mr McGee said the prison authorities had failed to check his false address or
bogus reference, from "a long-defunct
firm", before offering him a job.
Woodhill managers also failed to spot obvious signs of his profession, such
as the word "journalist" stamped in his passport which he used as proof of
"At each point in my investigation I felt certain that there must be a vetting procedure that would expose the flaws in my application. But it kept moving forward, stage to stage, with no trouble at all.
"The camera I smuggled in could easily have been a small explosive or a
knife," he wrote.
Shadow Home Secretary Oliver Letwin said the story gave him some concern over security at the jail.
"I don't suppose any system for checking on who you employ will be perfect, but this does sound as if this was an indication that the checking of data and references is not what it should be," he told the BBC.
"It is also odd that he was able to take so many photographs without any of his colleagues noticing."
A Prison Service spokesman said it was "very concerned" by the claims and had begun a top level inquiry.
"The deputy director general has commenced an investigation which will be
conducted by a senior manager from the prison service and will report to
ministers as soon as possible."
They don't pay prison officers anywhere near the wages they should do
Prison Officers' Association
Prison officers' representatives said the apparent security lapse was "shocking", and blamed decentralised recruitment and staff shortages caused by poor pay.
Brian Caton of the Prison Officers' Association
told BBC Radio 5 Live the prison service always used to ensure "the people working with prisoners were who they said they were, and were capable of carrying out their job in the proper manner.
"I'm disgusted actually that the prison service are not vetting in the way they did when they recruited centrally...
"This kind of work, when you are dealing with prisoners of such notoriety, should be carried out by trained and experienced officers, not ... a probationary prison officer," he said.
He said any prison officer enlisted since vetting procedures were changed should now be checked.
And he urged better pay and conditions for prison officers.
"There is very, very severe staffing shortages. They don't pay prison officers anywhere near the wages they should do."
Huntley was taken to hospital last week after apparently taking an overdose of anti-depressants in a suicide attempt - despite being on suicide watch.
The Prison Officers Association had also blamed that incident on staffing pressures, which meant
officers with no medical training were responsible for prisoners taking their medication.
Huntley denies murdering Holly Wells (left) and Jessica Chapman
Mr Huntley, 29, is on remand awaiting trial for the murders of 10-year-olds Jessica
Chapman and Holly Wells in Soham, Cambridgeshire, last year.
Mr Huntley, who denies two charges of murder, is due to go on trial in October with his girlfriend Maxine Carr, also of Soham.
She denies two offences of helping an offender and a charge of attempting to pervert the course of justice.